In industrial installations using large-size screwing elements, during tightening and loosening operations a pull is exerted on the elements by means of a tensioning device, in order to keep them in a pre-stress state.
This applies particularly to nuclear reactors, where a cover is fastened removably on a vessel, to permit periodic reloading of the reactor with fuel and, inspection of the vessel interior.
For this purpose, the cover is fastened on the reactor vessel by means of large-size pins which are screwed into the collar of the vessel and which penetrate into smooth holes provided in the collar of the cover. These pins are usually used in fairly large numbers and are relatively close to one another, with a result that, to remove and refit the cover, it is necessary to unscrew and rescrew all these pins, and also tension them during each screwing and unscrewing operation.
This tensioning of the pins during each screwing and unscrewing operation is usually carried out by an assembly of jacks exerting a pulling force on each pin by means of an engaging piece consisting, for example, of a tensioning nut screwed onto the free end of the pin.
In view of the considerable weight of the pins and tensioning nuts, the screwing of the nuts is a difficult operation, since it is possible to ensure that these various elements are coaxial only by trial and error, and this risks causing damage to the first thread as a result of the absence of any centering of the nut relative to the pin and the lack of angular adjustment. Consequently, each operation requires a new search in order to align the threads with one another. A relatively extensive period of time is required to carry out this operation and, as a result, the personnel performing the work risks remaining in a zone of excessively high radioactive radiation for far too long.
FR-A-2,367,574, discloses a device for fitting and removing pins, in which the nut used for tightening and prestressing the pin has, in its lower part, a threaded end piece with a finger limiting screwing onto the pin and, in its central part, a spherical stop interacting with a receptacle of matching shape. The upper end of the nut is connected to a system with a double cardan joint, and the assembly as a whole is associated with an elastic suspension.
In the stage of searching for the ideal screwing position, the kinematic chain resulting from the association of these elements gives the nut a ball-and-socket movement, thus giving rise to undesirable friction, mainly in the region of contact between the two spheres.